1. Field of the Invention
The invention relates to a process for preparing organosilicon compounds containing isocyanate groups from organosilicon compounds containing carbamato groups and having a low halide content.
2. Background Art
Organosilicon compounds containing isocyanate groups can be employed in numerous applications, for example as adhesion promoters, coupling agents, crosslinkers or surface modifiers in sealants and adhesives, and as coatings additives, among others.
Various processes for synthesizing organosilicon compounds containing isocyanate groups have already been described, such as the reaction of trichlorosilane with allyl isocyanate under Pt catalysis. The latter process, however, is characterized by poor yields, and requires expensive unsaturated isocyanates. In a similar process, trialkoxysilanes are employed in lieu of trichlorosilanes. However, the use of trialkoxysilanes with their attendant high hazard potential, i.e. disproportionation during storage, formation of spontaneously flammable gases, toxicity, etc., imposes extremely high requirements on the process regime, thereby imposing additional restrictions on its economics.
Silicon-containing amines have also been reacted with phosgene in the presence of various HCl acceptors in order to obtain the corresponding isocyanate compounds. In view of the extraordinarily high toxicity of phosgene, costly and inconvenient safety measures are of course necessary in order to protect people and the environment.
There is also mention in the patent literature of thermal cleavage of organosilicon compounds containing carbamate groups at temperatures starting at about 300° C. e.g., U.S. Pat. No. 5,393,910 A. In these pyrolysis or thermolysis reactions the desired organosilicon compounds containing isocyanate groups are formed with the release of equimolar amounts of an alcohol. However, in practice, it has been found difficult to produce organosilicon compounds containing isocyanate groups in a continuous fashion by such methods, due to buildup of deposits in the thermolysis apparatus. The yields tend to be low, and the product is prone to discoloration.